Proximity sensors are used in various industries to detect when an object is nearby without any physical contact. A proximity sensor often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for example), and looks for changes in the field or return signal. Various types of proximity sensors are available, including capacitive, magnetic and inductive. The proximity sensor is typically mounted to a base and the moving object being sensed is often referred to as the proximity sensor's target.
Inductive proximity sensors that operate by creating and sensing electromagnetic fields are particularly useful for sensing the presence/position of metallic objects, such as conveyors, lift tables and machine parts, while ignoring the presence of nonmetallic objects that may be positioned around the sensors. Although there are various manners of mounting inductive proximity sensors, a common manner of mounting such sensors is by inserting the sensor into an orifice within a metallic support structure such as a metallic wall. Typically, upon mounting an inductive proximity sensor in this manner, the sides of the sensors are in contact with the metallic wall and, in many circumstances, a front surface of the sensor that is intended to face a target is mounted flush with the surface of the surrounding metallic wall.
When mounted in this manner, an electromagnetic field created by the sensor can interact with the surrounding metal of the support structure, in addition to interacting with any target that may be present. Such an interaction with the surrounding metal is commonly known as “mounting effect”, and the strength of such an interaction can depend upon various factors including the composition of the surrounding metal and its position relative to the sensor and particularly the sensor's front surface. The mounting effect can result in a deterioration of the sensor's performance in terms of sensitivity or accuracy. For example, the mounting effect can in some circumstances precipitate “false triggering” (sensing of a target when it is not yet within a particular range of the sensor).
More particularly in this regard, an inductive proximity sensor may be designed so as to detect the presence of a target if the electromagnetic field sensed via an electromagnetic coil within the sensor varies more than a threshold amount from a base value that is representative of the electromagnetic field that would be sensed in the absence of a target. However, the assumed base value or threshold amount may be appropriate only when the sensor is mounted upon a particular metallic supporting structure (or when the sensor is operating entirely independently of any such structure). If this is the case, the assumed base value or threshold amount may no longer be appropriate once the sensor is mounted upon a different metallic supporting structure, and thus experiences a different mounting effect, than what was assumed to be the case.
Various efforts have been made to combat the mounting effect and its negative consequences. In particular with respect to false triggering, some have attempted to mitigate this problem by employ a higher threshold, so that the mounting effect will not cause inappropriate meeting of the threshold as frequently. Unfortunately, adjusting the threshold level higher while maintaining the same measurement base value results in the sensor having a lower sensitivity and a reduced sensing distance.
Another manner of reducing the mounting effect that has been attempted involves the use of shielding rings. By wrapping an inductive proximity sensor in a metal ring (for example, a copper ring), it is often the case that the ring will suppress the mounting effect by reducing the magnetic field leakage between the sensor and the supporting structure. However, this method is unsatisfactory, since it cannot completely eliminate the leakage magnetic field and therefore the mounting effect still exists, and also since the suppression of the mounting effect is achieved at the cost of a reduced magnetic field at the target position, such that the sensitivity and sensing distance of the sensor is reduced. Still another method that has been tried involves compensating the mounting effect by manually adjusting certain sensor parameters (e.g., the sensor's base value) after it is mounted. Although this method can reduce the mounting effect, it is not widely implemented because it requires a user's additional manual commissioning.
For at least these reasons, therefore, it would be advantageous if an improved inductive proximity sensor could be developed in which the performance of the sensor was less susceptible to, and less diminished by, the mounting effect. More particularly, in at least some embodiments, it would be advantageous if such an improved sensor could achieve such levels of performance without experiencing (at least to the same degree) one or more of the above-described disadvantages associated with conventional methods of dealing with the mounting effect.